The present disclosure relates generally to exhaust emission control devices such as automotive catalytic converters. More particularly, the disclosure relates to a method of manufacturing emission control devices having fragile substrates.
Exhaust emission control devices comprise catalytic converters, evaporative emissions devices, scrubbing devices (e.g., hydrocarbon, sulfur, and the like), particulate filters/traps, adsorbers/absorbers, as well as combinations comprising at least one of the foregoing devices. This disclosure relates to such emission control devices having a ceramic substrate disposed within a metal housing with a matting material disposed between the substrate and housing. Although the substrate can have any size or geometry, the size and geometry are preferably chosen to optimize surface area in the given converter design parameters. Typically, the substrate has a honeycomb geometry, with the combs through-channel having any multi-sided or rounded shape, with substantially square, triangular, pentagonal, hexagonal, heptagonal, or octagonal or similar geometries preferred due to case of manufacturing and increased surface area. Although converter housings are most often cylindrical, they can be other shapes as well, such as having elliptical, oval, polygonal, and other cross sections.
In cases where a chemical washcoat is applied to the substrate, for example, a catalyst material and/or an adsorption material washcoat layer, the materials are wash coated, imbibed, impregnated, physisorbed, chemisorbed, precipitated, or otherwise applied to the catalyst substrate. In addition, a binder and/or support material such as aluminum oxide can be mixed with the chemical washcoat according to the specification of the particular application. Once the washcoat is applied to the substrate, the substrate is fired at a high temperature to sinter the washcoat thereby forming a hard phase of the chemical coating. It has previously been known to fire the washcoat prior to assembling the substrate into the emission control device housing.
Newer “thin-wall” substrates used in exhaust emission control devices offer significant advantages over traditional substrates, including a greater geometric surface area per unit volume and faster catalyst light-off due to the lower thermal mass of the substrate. As is generally understood in the art, faster light-off translates to higher conversion efficiency since catalytic converters are not effective during a cold engine start until they reach operation, or light-off temperature. However, thin wall substrates are significantly more fragile and are subject to fracture during stressful manufacturing operations that including stuffing, sizing, and burnoff operations.
One method currently employed to manufacture an emission control devices include wrapping the substrate in a retention material, or mat, and stuffing the substrate and mat into a metal housing through the use of a stuffing cone, the cone serving to compress the matting so that it can slide into the housing (see FIG. 2). The retention material enhances the structural integrity of the substrate by applying compressive radial forces about it, reducing its axial movement and retaining it in place. Typically, the retention material is concentrically disposed around the substrate to form a retention material/substrate subassembly.
Another method involves disposing particulate retention material between the substrate and the housing. While the retention material can be in the form of a mat, particulates, or the like, it will be referred to herein simply as “mat” and it will be understood that the term “mat” as used herein refers to the retention material regardless of what form it is in.
Depending on the type of mat, whether it is intumescent or non-intumescent, the necessary stuffing pressure varies. Intumescent mats are called such because they swell under high temperature. This swelling is a property of a component of the mat, typically vermiculite. Non-intumescent mats do not contain vermiculite. These materials can comprise ceramic materials and other materials such as organic binders and the like, or combinations comprising at least one of the foregoing materials. Non-intumescent materials include materials such as those sold under the trademarks “NEXTEL” and “SAFFIL” by the “3M” Company, Minneapolis, Minn., or those sold under the trademark, “FIBERFRAX” and “CC-MAX” by the Unifrax Co., Niagara Falls, N.Y., and the like. Intumescent materials include materials sold under the trademark “INTERAM” by the “3M” Company, Minneapolis, Minn., as well as those intumescents which are also sold under the aforementioned “FIBERFRAX” trademark, as well as combinations thereof and others.
The swelling property of intumescent mats is useful because it helps to maintain a positive pressure between the substrate and the housing during the thermal cycle imposed on the converter in normal use without having to greatly compress the mat during manufacture. In use, the diameter of the metal housing increases due to thermal expansion to a greater degree than that of the ceramic substrate. Thus, to maintain a positive pressure, a mat must expand to fill the growing gap as the temperature rises. Non-intumescent mats must be stuffed under much greater force to a high level of compression in order to ensure a continued positive pressure between the substrate and housing during use. This high-force stuffing is more time consuming and takes considerable energy, which significantly increases the overall production cost of the converter.
Once the substrate and matting material is stuffed into the housing, the housing may be sized and appropriate connections are formed for assembly into an exhaust system. Sizing operations, when necessary, compensate for variations in substrate diameters, and may comprise compressing the housing to produce an overlapped seam, and then welding, or a housing may be reduced by drawing or compressing the housing using a pipe-sizer.
Exhaust pipe connections may be formed in or welded onto either end of the housing. The ends of the shell may be sized, e.g., using a spinform method, to form a conical shaped inlet and/or a conical shaped outlet, thus eliminating the need for separate endcone assemblies in at least one embodiment of the exhaust emission control device. In the alternative, one or both ends of the shell can also be sized so that an end cone, an end plate, an exhaust gas manifold assembly, or other exhaust system component, and combinations comprising at least one of the foregoing components, can be attached to provide a gas tight seal. Thus, the connections include portions having varying cross-sections to conform the stream entering the converter to the shape of the substrate, thereby allowing exhaust to flow from the engine into and out of the converter, and through the remaining exhaust system to the tail pipe.
After the exhaust pipe connections are formed on the housing, the emission control device is ready to be assembled into an engine. During the converter's first use, the converter is heated to normal operating temperature, which may be anywhere from 300° C. to more than 500° C. This first use or heating drives off organic binders within the mat and causes the intumescent material within the mat to greatly expand, thus increasing the pressure within the confines between the housing and substrate. Some substrates, particularly the newer, more fragile substrates, can fail under this pressure, rendering the entire converter unusable.
For comparision, a graph showing estimated matting pressure as a function of type of operation during the prior art manufacturing method described above is provided in FIG. 6. Starting at the left side of the graph, the intumescent matting is stuffed under very low pressure, i.e., less than 10 pounds per square inch (psi). The pressure is greatly increased to about 150 psi during the sizing operation. After sizing, the matting responds by relaxing somewhat, reducing the stress therein and the pressure to about 100 psi. Finally, the converter is heated, e.g., during its first use, which causes swelling of the matting, which increases the pressure by about 80 psi to 180 psi. The pressure may be even higher locally within the matting material due to variations in the matting or the substrate itself. The fragile ceramic substrate is sometimes unable to stand up to these high pressures and fails.
It would be desirable to reduce the likelihood of breaking the ceramic substrate during the production or first use of a catalytic converter.